The field of the invention is that of heat treatments of castings made in an alloy based on aluminium.
The invention more specifically relates to a method for the heat treatment of cylinder heads molded in aluminium alloys and to the cylinder heads resulting from such a method.
Cylinder heads molded in aluminium alloys are in vast majority used in performing automobile engines.
The increase in the specific powers of these engines, and the search for better performances as regards polluting emissions leading to increased pressure on the material of the cylinder head, one skilled in the art conventionally uses structurally hardened casting alloys.
Aluminium casting alloys include different families of composition, most of which are capable of structural hardening by heat treatment. In particular, mention may be made of the aluminium/silicon/magnesium family and of the aluminium/silicon/copper/magnesium family.
Notably, for making automobile components in mass production, for example cylinder heads subject to very strong loads upon use, one skilled in the art notably uses:
alloys of the A319 type according to the designation of the Aluminium Association, comprising from 5.0% to 9.0% of silicon, from 2.0% to 3.5% of copper, from 0.2 to 0.6% of magnesium,
alloys of the A356 or A357 type according to the designation of the Aluminium Association, comprising from 6.5% to 7.5% of silicon, from 0.2 to 0.7% of magnesium,
or even intermediate alloys or alloys close to both of these families.
In particular, alloys of the A356 type further comprising 0.56% copper are frequently used for heavy duty diesel engine cylinder heads.
In order to maximize the mechanical properties of these alloys, it is customary use, at least for the cases of the most severe loads, to achieve full heat treatment associating in this order:
high temperature solution heat treatment, typically between 490° C. and 550° C.;
water quenching between the solutionization temperature and the water quenching temperature (typically between 20° C. and 95° C.; boiling water may also be used);
cooling in the surroundings of the workshop before a tempering treatment for hardening at temperatures of the order of 120° C. to 250° C.
The above treatments are typically carried out over dwelling times of the order of 5 to 6 hours at a temperature in the oven, the rise in temperature of the load usually being of the order of 1 to 2 hours for cylinder heads.
However, these practices for parts with a geometry as complex as that of cylinder heads lead during the quenching to the generation of significant residual stresses which may locally reach the cold elastic limit level of the material.
Increasing the temperature of the quenching water is a well-known means to one skilled in the art for reducing these residual stresses.
However, the use of boiling water does not allow complete suppression of these residual stresses. Further, quenching with boiling water leads to significant reductions in mechanical characteristics.
One skilled in the art partly finds a remedy to the persistence of residual stresses after quenching by practicing hardening tempering operations at high temperatures, typically beyond 200° C., leading in this way the alloy to an over-tempering state (transition to beyond the maximum strength peak). These types of treatments are commonly called T7 treatments.
It has also been proposed to reduce the time required for achieving the whole of the heat treatment (solutionization, quenching and tempering). Document GB 2,361,710 thus proposes to limit the duration of solutionization to a maximum of three hours, and preferably to a maximum of two hours. The treatment recommended by this document, for a shorter treatment time, results in mechanical characteristics similar to those of the T7 treatment.
It will be noted that this document GB 2,361,710 specifies that the fatigue properties are only very slightly influenced by the type of heat treatment and indicates that owing to this, the fatigue properties of parts subject to the T7 treatment or to the treatment according to this document GB 2,361,710 with shorter solutionization should not be significantly different.
Document U.S. Pat. No. 6,752,885 also provides a shorter heat treatment than the T7 treatment, this treatment comprising solutionization for a duration comprised between one hour thirty minutes and two hours and tempering at a temperature of 250° C.
The applicant has also developed and has been commercially using for many years a treatment of the T5 type consisting of the following sequence:
fast solidification inside the mold, generally a metal mold, at least for the portion molding the fire face of the cylinder head,
fast cooling with forced air after ejecting the part out of the mold, the part then being still provided with its inner sand cores and with its feeder heads, down to room temperature,
the whole being followed by hardening tempering.
This treatment has the advantage of suppressing residual stresses, but with it, it is not possible to draw the whole benefit from the hardening potential of the alloy.